92 ACURA VIGOR MYSTERY PROBLEM!!

Have owned this Vigor for 3 years with great luck relative to any engine problems, etc. Recently, the engine started missing, backfiring, & just running lousy. The mystery is it runs/purrs like a cat upon a cold start & continues to run great until the thermostat opens up; then it starts missing, backfiring, & engine dies then won't start again until it cools down completely???? I replaced the "engine coolant sensor" as I found out this sensor tells the ECU to richen the fuel/air mixture when the engine is cold, then leans it out as it reaches normal operating temp. When this sensor fails; it is locked in the cold engine mode which floods the engine after warm-up. Also changed the plugs and plug wires with no improvement at all!! The problem still exists & my local independant shop mechanic is scratching his head too. No "Check Engine" light is on either? Any help is most appreciated as the closest Acura/Honda dealer is over 100 miles away from where I live.